Thin-film transistor having hydrogen-blocking layer and display apparatus including the same

ABSTRACT

A thin-film transistor is disclosed. The thin-film transistor includes an oxide semiconductor layer disposed on a substrate, a gate electrode disposed so as to overlap at least a portion of the oxide semiconductor layer and isolated from the oxide semiconductor layer, a source electrode connected to the oxide semiconductor layer, and a drain electrode connected to the oxide semiconductor layer and spaced apart from the source electrode, wherein the oxide semiconductor layer includes a first sub layer disposed on the substrate, a second sub layer disposed on the first sub layer, and a third sub layer disposed on the second sub layer, the second sub layer has larger resistance than the first sub layer and the third sub layer and lower carrier concentration than the first sub layer and the third sub layer, the first sub layer has higher hydrogen concentration than the second sub layer and the third sub layer, and each of the first sub layer and the second sub layer has crystallinity.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority of Korean Patent Application No.10-2017-0148778 filed in the Republic of Korea on Nov. 9, 2017, which ishereby incorporated by reference in its entirety.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The present disclosure relates to a thin-film transistor having ahydrogen-blocking layer, a method of manufacturing the thin-filmtransistor, and a display apparatus including the thin-film transistor.

Description of the Background

In the field of electronic equipment, a transistor has been widely usedas a switching device or a driving device. In particular, a thin-filmtransistor has been widely used as a switching device of a displayapparatus, such as a liquid crystal display apparatus or an organiclight-emitting display apparatus, since the thin-film transistor can bemanufactured on a glass substrate or a plastic substrate.

Based on a material constituting an active layer, the thin-filmtransistor may be classified as an amorphous silicon thin-filmtransistor, in which amorphous silicon is used as the active layer, apolycrystalline silicon thin-film transistor, in which polycrystallinesilicon is used as the active layer, or an oxide semiconductor thin-filmtransistor, in which an oxide semiconductor is used as the active layer.

The amorphous silicon thin-film transistor (a-Si TFT) has advantages inthat manufacturing time is short and the manufacturing cost is low,since the amorphous silicon is deposited within a short time in order toform the active layer. However, the amorphous silicon thin-filmtransistor has disadvantages in that the amorphous silicon thin-filmtransistor has low Hall mobility, whereby the current-driving ability ofthe amorphous silicon thin-film transistor is not good, and that thethreshold voltage of the amorphous silicon thin-film transistor ischanged, whereby the use of the amorphous silicon thin-film transistorin an active matrix organic light-emitting device (AMOLED) is limited.

The polycrystalline silicon thin-film transistor (poly-Si TFT) ismanufactured by depositing and crystallizing amorphous silicon. Sincethe process of crystallizing amorphous silicon is required in order tomanufacture the polycrystalline silicon thin-film transistor, the numberof processes is increased, with the result that manufacturing cost isincreased. In addition, since the crystallizing process is performed ata high process temperature, it is difficult to apply the polycrystallinesilicon thin-film transistor to a large-sized apparatus. Furthermore, itis difficult to secure the uniformity of the polycrystalline siliconthin-film transistor due to the polycrystalline properties thereof.

For the oxide semiconductor thin-film transistor (oxide semiconductorTFT), an oxide constituting the active layer may be deposited at arelatively low temperature, the Hall mobility of the oxide semiconductorthin-film transistor is high, and a change in the resistance of theoxide semiconductor thin-film transistor is great depending on thecontent of oxygen, whereby desired physical properties of the oxidesemiconductor thin-film transistor are easily obtained. In addition, theoxide semiconductor thin-film transistor is advantageous in therealization of a transparent display, since the oxide semiconductor istransparent due to the properties of the oxide. However, oxygen vacancyoccurs in the oxide semiconductor due to the permeation of hydrogencaused by contact of the oxide semiconductor with an insulation layer ora passivation layer, whereby the reliability of the oxide semiconductormay be reduced.

In particular, a plastic substrate, such as a polyimide (PI) substrate,contains a large amount of hydrogen. Therefore, in the case in which anoxide semiconductor layer is formed on a flexible substrate, the oxidesemiconductor layer can be damaged by a large amount of hydrogendischarged from the plastic substrate. In order to prevent this, abuffer layer can be disposed on the plastic substrate, and the oxidesemiconductor layer can be formed on the buffer layer. Even in thiscase, the oxide semiconductor layer can be damaged by hydrogen containedin the buffer layer. For this reason, it is difficult to stably form theoxide semiconductor layer on the plastic substrate, such as a polyimide(PI) substrate. The above described background is disclosed in KoreanPatent Application Publication No. 10-2017-0024130 entitledSEMICONDUCTOR DEVICE AND METHOD OF MANUFACTURING THE SAME and KoreanPatent Application Publication No. 10-2015-0061076 entitled ARRAYSUBSTRATE AND METHOD OF MANUFACTURING THE SAME.

SUMMARY OF THE DISCLOSURE

The present disclosure has been made in view of the above problems, andis to provide a thin-film transistor including a hydrogen-blocking layerthat exhibits an excellent hydrogen-blocking property.

In addition, the present disclosure to provide a thin-film transistorincluding a hydrogen-blocking layer that exhibits an excellenthydrogen-blocking property and an oxide semiconductor layer that is notdamaged even when formed on a plastic substrate, such as a polyimide(PI) substrate.

Moreover, the present disclosure to provide a display apparatusincluding the thin-film transistor described above.

Further, the present disclosure to provide a flexible display apparatusincluding the thin-film transistor described above.

In accordance with an aspect of the present disclosure, the above andother objects can be accomplished by the provision of a thin-filmtransistor including an oxide semiconductor layer disposed on asubstrate, a gate electrode disposed so as to overlap at least a portionof the oxide semiconductor layer and isolated from the oxidesemiconductor layer, a source electrode connected to the oxidesemiconductor layer, and a drain electrode connected to the oxidesemiconductor layer and spaced apart from the source electrode, whereinthe oxide semiconductor layer includes a first sub layer, a second sublayer and a third sub layer, which are sequentially disposed, the secondsub layer has larger resistance than the first sub layer and the thirdsub layer and lower carrier concentration than the first sub layer andthe third sub layer, the first sub layer has higher hydrogenconcentration than the second sub layer and the third sub layer, andeach of the first sub layer and the second sub layer has crystallinity.

Each of the first sub layer and the second sub layer may have C-axiscrystallinity and a wurtzite crystal structure.

The first sub layer and the second sub layer may have the same metalelement composition.

The third sub layer may have no C-axis crystallinity.

Each of the first sub layer, the second sub layer, and the third sublayer may include indium (In), gallium (Ga), and zinc (Zn), and thethird sub layer may have higher indium (In) concentration (at %) thanthe first sub layer and the second sub layer.

The content of indium (In), gallium (Ga), and zinc (Zn) in each of thefirst sub layer and the second sub layer may be set so as to satisfyEquations 1 and 2 below.2≤[Ga]/[In]≤4  [Equation 1]2≤[Zn]/[In]≤6  [Equation 2]

In Equations 1 and 2, [Ga] indicates the number of atoms in gallium(Ga), [In] indicates the number of atoms in indium (In), and [Zn]indicates the number of atoms in zinc (Zn).

The first sub layer may have a larger taper angle than the third sublayer.

The second sub layer may have a thickness equivalent to 1 to 10 timesthe thickness of the first sub layer.

The first sub layer may have a thickness ranging from 5 to 15 nm, andthe second sub layer may have a thickness ranging from 15 to 50 nm.

The third sub layer may have a first conductivized portion formed at theregion thereof that does not overlap the gate electrode.

The second sub layer may have a second conductivized portion formed atthe region thereof that does not overlap the gate electrode so as tocontact the first conductivized portion conductivized portion.

The second conductivized portion may not contact the first sub layer.

The substrate may be a plastic substrate.

The thin-film transistor may further include a buffer layer disposedbetween the substrate and the oxide semiconductor layer and alight-blocking layer disposed between the substrate and the buffer layerso as to overlap the oxide semiconductor layer.

The thin-film transistor may further include a gate insulation filmdisposed between the oxide semiconductor layer and the gate electrode,wherein the oxide semiconductor layer may be disposed so as to be closerto the substrate than the gate electrode on the basis of the gateinsulation film.

The thin-film transistor may further include a gate insulation filmdisposed between the oxide semiconductor layer and the gate electrode,wherein the gate electrode may be disposed so as to be closer to thesubstrate than the oxide semiconductor layer on the basis of the gateinsulation film.

The first sub layer may be disposed so as to be closer to the substratethan the third sub layer on the basis of the second sub layer.

The third sub layer may be disposed so as to be closer to the substratethan the first sub layer on the basis of the second sub layer.

In accordance with another aspect of the present disclosure, there isprovided a display apparatus including a substrate, a thin-filmtransistor disposed on the substrate, and a first electrode connected tothe thin-film transistor, wherein the thin-film transistor includes anoxide semiconductor layer disposed on the substrate, a gate electrodedisposed so as to overlap at least a portion of the oxide semiconductorlayer and isolated from the oxide semiconductor layer, a sourceelectrode connected to the oxide semiconductor layer, and a drainelectrode connected to the oxide semiconductor layer and spaced apartfrom the source electrode, and wherein the oxide semiconductor layerincludes a first sub layer, a second sub layer and a third sub layer,which are sequentially disposed, the second sub layer has largerresistance than the first sub layer and the third sub layer and lowercarrier concentration than the first sub layer and the third sub layer,the first sub layer has higher hydrogen concentration than the secondsub layer and the third sub layer, and each of the first sub layer andthe second sub layer has crystallinity.

Each of the first sub layer and the second sub layer may have C-axiscrystallinity and a wurtzite crystal structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of thepresent disclosure will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a cross-sectional view of a thin-film transistor according toan aspect of the present disclosure;

FIGS. 2A and 2B are schematic views showing a process of forming a firstsub layer and a second sub layer;

FIG. 3 is a schematic view showing a wurtzite crystal structure;

FIG. 4 is an enlarged view showing part A of FIG. 1;

FIG. 5 is a cross-sectional view of a thin-film transistor according toanother aspect of the present disclosure;

FIGS. 6A and 6B are cross-sectional views of thin-film transistorsaccording to other aspects of the present disclosure;

FIGS. 7A and 7B are cross-sectional views of thin-film transistorsaccording to other aspects of the present disclosure;

FIG. 8 is a schematic cross-sectional view of a display apparatusaccording to another aspect of the present disclosure;

FIG. 9 is a schematic cross-sectional view of a display apparatusaccording to a further aspect of the present disclosure;

FIGS. 10A and 10B are transmission electron microscope (TEM) photographsrespectively showing a third sub layer and a bulk layer;

FIGS. 11A and 11B are transmission electron microscope (TEM) photographsrespectively showing taper shapes formed at oxide semiconductor layersaccording to Comparative Example 1 and Example 1;

FIG. 12 is a graph showing the content of hydrogen in each oxidesemiconductor layer based on the depth thereof; and

FIG. 13 is a graph showing the average content of hydrogen contained ina third sub layer of each of oxide semiconductor layers according toComparative Examples 1 to 4 and Examples 1 to 3.

DETAILED DESCRIPTION

Advantages and features of the present disclosure, and implementationmethods thereof will be clarified through the following aspects,described with reference to the accompanying drawings. The presentdisclosure may, however, be embodied in different forms and should notbe construed as being limited to the aspects set forth herein. Rather,these aspects are provided so that this disclosure will be thorough andcomplete, and will fully convey the scope of the present disclosure tothose skilled in the art. Further, the present disclosure is onlydefined by the scope of the claims.

The shapes, sizes, ratios, angles, and numbers disclosed in the drawingsfor describing aspects of the present disclosure are merely examples,and thus the present disclosure is not limited to the illustrateddetails. Like reference numerals refer to like elements throughout. Inthe following description, when the detailed description of the relevantknown function or configuration is determined to unnecessarily obscurethe important point of the present disclosure, the detailed descriptionwill be omitted.

In the case in which “comprise”, “have”, and “include” described in thepresent specification are used, another part may also be present unless“only” is used. The terms in a singular form may include plural formsunless noted to the contrary.

In construing an element, the element is construed as including an errorregion although there is no explicit description thereof.

In describing a positional relationship, for example, when thepositional order is described as “on”, “above”, “below”, and “next”, thecase of no contact therebetween may be included, unless “just” or“direct” is used. If it is mentioned that a first element is positioned“on” a second element, it does not mean that the first element isessentially positioned above the second element in the figure. The upperpart and the lower part of an object concerned may be changed dependingon the orientation of the object. Consequently, the case in which afirst element is positioned “on” a second element includes the case inwhich the first element is positioned “below” the second element as wellas the case in which the first element is positioned “above” the secondelement in the figure or in an actual configuration.

Spatially relative terms such as “below”, “beneath”, “lower”, “above”,or “upper” may be used herein to describe a relationship of a device oran element to another device or another element as shown in the figures.It will be understood that spatially relative terms are intended toencompass different orientations of a device during the use or operationof the device, in addition to the orientation depicted in the figures.For example, if a device in one of the figures is turned upside down,elements described as “below” or “beneath” other elements would then beoriented “above” the other elements. The exemplary term “below” or“beneath” can, therefore, encompass both an orientation of below andabove. In the same manner, the exemplary term “above” or “upper” canencompass both an orientation of above and below.

In describing a temporal relationship, for example, when the temporalorder is described as “after”, “subsequent”, “next”, and “before”, acase which is not continuous may be included, unless “just” or “direct”is used.

It will be understood that, although the terms “first”, “second”, etc.may be used herein to describe various elements, these elements shouldnot be limited by these terms. These terms are only used to distinguishone element from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure.

The terms “first horizontal axis direction”, “second horizontal axisdirection”, and “vertical axis direction” should not be interpreted onlybased on a geometrical relationship in which the respective directionsare perpendicular to each other, and may be meant as directions havingwider directivities within the range within which the components of thepresent disclosure can operate functionally.

It should be understood that the term “at least one” includes allcombinations related with any one item. For example, “at least one amonga first element, a second element and a third element” may include allcombinations of two or more elements selected from the first, second andthird elements as well as each element of the first, second and thirdelements.

Features of various aspects of the present disclosure may be partiallyor overall coupled to or combined with each other, and may be variouslyinter-operated with each other and driven technically as those skilledin the art can sufficiently understand. The aspects of the presentdisclosure may be carried out independently from each other, or may becarried out together in a co-dependent relationship.

Hereinafter, a thin-film transistor and a display apparatus includingthe same according to aspects of the present disclosure will bedescribed in detail with reference to the accompanying drawings. In thedrawings, the same or similar elements are denoted by the same referencenumerals even though they are depicted in different drawings.

FIG. 1 is a cross-sectional view of a thin-film transistor 100 accordingto an aspect of the present disclosure.

The thin-film transistor 100 according to the aspect of the presentdisclosure includes an oxide semiconductor layer 130 disposed on asubstrate 110, a gate electrode 140 disposed so as to overlap at least aportion of the oxide semiconductor layer 130 and isolated from the oxidesemiconductor layer 130, a source electrode 150 connected to the oxidesemiconductor layer 130, and a drain electrode 160 connected to theoxide semiconductor layer 130 and spaced apart from the source electrode150.

Glass or plastic may be used as the substrate 110. Transparent plasticthat exhibits flexibility, such as polyimide (PI), may be used as theplastic.

In the case in which polyimide (PI) is used as the substrate 110,heat-resistant polyimide, which withstands high temperatures, may beused in consideration of the fact that a high-temperature depositionprocess is carried out on the substrate 110. In this case, processes,such as deposition and etching, may be carried out in the state in whichthe polyimide substrate is disposed on a carrier substrate composed of ahighly durable material, such as glass, in order to form the thin-filmtransistor 100.

In addition to the polyimide substrate, other plastic substrateswell-known in the art may be used. For example, a polycarbonate (PC)substrate, a polyethersulfone (PES) substrate, a polyethylenenaphthalate (PEN) substrate, a polyethylene terephthalate (PET)substrate, or a polystyrene (PS) substrate may be used as the substrate110 of FIG. 1.

More specifically, a substrate 110 according to an aspect of the presentdisclosure may include at least one of polyimide (PI), polycarbonate(PC), polyethersulfone (PES), polyethylene naphthalate (PEN),polyethylene terephthalate (PET), or polystyrene (PS).

Such a plastic substrate may be used to manufacture a flexible displayapparatus. The thin-film transistor 100 according to the aspect of thepresent disclosure may be formed on a flexible substrate such that thethin-film transistor can be used as a driving or switching thin-filmtransistor of a flexible display apparatus.

The plastic substrate contains a larger amount of hydrogen than a glasssubstrate. During a process of manufacturing the thin-film transistor100 or during a process of manufacturing a display apparatus 500 or 600,the hydrogen may leak from the plastic substrate, whereby the othercomponents of the thin-film transistor 100 or the display apparatus 500or 600 may be affected.

For example, the hydrogen contained in the plastic substrate may move tothe oxide semiconductor layer 130, and may be coupled to oxygen in theoxide semiconductor layer 130, whereby oxygen vacancy may occur in theoxide semiconductor layer 130, or the oxide semiconductor layer 130 maybe conductivized. In the case in which the hydrogen (H) contained in theplastic substrate moves to the oxide semiconductor layer 130, asdescribed above, the oxide semiconductor layer 130 is damaged, wherebythe reliability of the thin-film transistor 100 is reduced.

In order to protect the oxide semiconductor layer 130 or the thin-filmtransistor 100 from hydrogen (H), oxygen (O₂), or moisture (H₂O)discharged from the plastic substrate or introduced from the outside, abuffer layer (not shown) may be disposed on the substrate 110.

However, the thin-film transistor 100 according to the aspect of thepresent disclosure has a first sub layer 131 provided in the oxidesemiconductor layer 130. The first sub layer 131 serves as ahydrogen-blocking layer, whereby the buffer layer may be omitted.

Referring to FIG. 1, the oxide semiconductor layer 130 is disposed onthe substrate 110.

The oxide semiconductor layer includes a first sub layer, a second sublayer and a third sub layer, which are sequentially disposed. Forexample, the oxide semiconductor layer 130 includes a first sub layer131 disposed on the substrate 110, a second sub layer 132 disposed onthe first sub layer 131, and a third sub layer 133 disposed on thesecond sub layer 132.

According to an aspect of the present disclosure, a channel of thethin-film transistor 100 is formed in the third sub layer 133.Consequently, the third sub layer 133 is called a channel layer. Thethird sub layer 133 includes an oxide semiconductor material. Forexample, the third sub layer 133 may be made of an oxide semiconductormaterial, such as an InZnO (IZO)-based oxide semiconductor material, anInGaO (IGO)-based oxide semiconductor material, an InSnO (ITO)-basedoxide semiconductor material, an InGaZnO (IGZO)-based oxidesemiconductor material, an InGaZnSnO (IGZTO)-based oxide semiconductormaterial, a GaZnSnO (GZTO)-based oxide semiconductor material, or anInSnZnO (ITZO)-based oxide semiconductor material. However, the presentdisclosure is not limited thereto. The third sub layer 133 may be madeof any of other oxide semiconductor materials well-known in the art.

The second sub layer 132 is disposed between the first sub layer 131 andthe third sub layer 133 in order to isolate the first sub layer 131 andthe third sub layer 133 from each other. The second sub layer 132 servesas a support for supporting the first sub layer 131. In addition, aportion of the region of the second sub layer 132 that is adjacent tothe third sub layer 133 may serve as a channel.

The first sub layer 131 serves as a hydrogen-blocking layer forpreventing the introduction of hydrogen (H) into the third sub layer133, which serves as a channel layer. The first sub layer 131 protectsthe third sub layer 133, which serves as a channel layer, from hydrogen.More specifically, the first sub layer 131 serves as a barrier forblocking hydrogen (H) from being introduced into the third sub layer133.

The first sub layer 131 and the second sub layer 132 may be made of thesame oxide semiconductor material. More specifically, a bulk layer 130Bfor forming the first sub layer 131 and the second sub layer 132 isformed using the same oxide semiconductor material, and then the firstsub layer 131 may be formed by introduction of hydrogen that isdischarged from the substrate 110 or introduced from the externalenvironment.

FIGS. 2A and 2B are schematic views showing a process of forming thefirst sub layer 131 and the second sub layer 132.

Referring to FIG. 2A, first, a bulk layer 130B for forming the first sublayer 131 and the second sub layer 132 is formed using the same oxidesemiconductor material. The bulk layer 130B may be formed by depositionand patterning. For example, the bulk layer 130B may be formed bysputter deposition. Alternatively, the bulk layer 130B may be formed bymetal organic chemical vapor deposition (MOCVD). The bulk layer 130Bformed by metal organic chemical vapor deposition (MOCVD) may have adense film structure.

Referring to FIG. 2A (the left part thereof), hydrogen (H) dischargedfrom the substrate 110 is introduced into the lower part of the bulklayer 130B during the process of manufacturing the thin-film transistor100. At this time, hydrogen (H) introduced from the outside or hydrogen(H) discharged from another insulation layer may be introduced into thelower part of the bulk layer 130B. A hydrogen-containing film having asmall thickness is formed in the lower part of the bulk layer 130Bthrough the introduction of hydrogen (H), whereby the first sub layer131 is formed (see the right part of FIG. 2A). As a result, the firstsub layer 131 and the second sub layer 132 are distinguished from eachother, although the first sub layer 131 and the second sub layer 132 mayhave same metal element composition mixed in same atomic ratio.

However, the present disclosure is not limited thereto. Referring toFIG. 2B, hydrogen is introduced into the bulk layer 130B from aninterlayer insulation film 170, which is a insulation film located onthe bulk layer 130B on the basis of the figure. As a result, ahydrogen-containing film having a small thickness may be formed in theupper part of the bulk layer 130B, whereby the first sub layer 131 maybe formed.

The first sub layer 131 formed as described above has a stable filmstructure that is capable of blocking hydrogen. Consequently, the firstsub layer 131 may also be referred to as a hydrogen-blocking layer.

According to an aspect of the present disclosure, the second sub layer132 has larger resistance than the first sub layer 131 and the third sublayer 132, and has lower carrier concentration than the first sub layer131 and the third sub layer 132.

Specifically, the first sub layer 131 is made of the same oxidesemiconductor material as the second sub layer 132, and is completedthrough the introduction of hydrogen. Consequently, the first sub layer131 has smaller resistance than the second sub layer 132, and has highercarrier concentration than the second sub layer 132. Meanwhile, thethird sub layer 133 serves as a channel layer. In order to serve as achannel layer, the third sub layer 133 is designed to have smallerresistance than the second sub layer 132 and to have higher carrierconcentration than the second sub layer 132. To this end, the third sublayer 133 may be made of an oxide semiconductor material different fromthe oxide semiconductor material of the first sub layer 131 and thesecond sub layer 132.

In addition, the first sub layer 131 blocks hydrogen from beingintroduced into the second sub layer 132 and the third sub layer 133,even though the first sub layer 131 is formed through the introductionof hydrogen. Consequently, the first sub layer 131 has higher hydrogenconcentration than the second sub layer 132 and the third sub layer 133.

Each of the first sub layer 131 and the second sub layer 132 hascrystallinity. More specifically, each of the first sub layer 131 andthe second sub layer 132 has C-axis crystallinity and a wurtzite crystalstructure. In the case in which the bulk layer 130B for forming thefirst sub layer 131 and the second sub layer 132, shown in FIGS. 2A and2B, includes a larger amount of zinc (Zn) than indium (In) based on thenumber of atoms, and is formed by deposition, such as sputterdeposition, at a predetermined temperature, the bulk layer 130B may haveC-axis crystallinity and a wurtzite crystal structure. As a result, eachof the first sub layer 131 and the second sub layer 132 may also haveC-axis crystallinity and a wurtzite crystal structure.

FIG. 3 is a schematic view showing a wurtzite crystal structure. Thewurtzite crystal structure is one of the crystal structures of an A-Btype compound, which is a hexagonal lattice in which a tetrahedralformation is formed around each of element A and element B.

The hydrogen (H) introduced into the lower part of the bulk layer 130B,which has C-axis crystallinity and a wurtzite crystal structure,permeates about 5 to 15 nm from the lower surface of the bulk layer130B, but does not move into the bulk layer 130B any further, whereby astable film is formed. That is, the hydrogen (H) introduced into thelower part of the bulk layer 130B forms a stable film together withexisting components of the bulk layer 130B, whereby the first sub layer131 is formed.

As a result, the first sub layer 131 may have a thickness ranging from 5to 15 nm. However, the present disclosure is not limited thereto. Thefirst sub layer 131 may have a thickness of more than 15 nm.

The first sub layer 131, formed as described above, may have a hydrogenconcentration equivalent to 10 times or more the hydrogen concentrationof the second sub layer 132. More specifically, the first sub layer 131may have a hydrogen concentration equivalent to 10 to 100 times thehydrogen concentration of the second sub layer 132.

According to an aspect of the present disclosure, the first sub layer131 and the second sub layer 132 may have the same metal elementcomposition. Referring to FIGS. 2A and 2B, the first sub layer 131 andthe second sub layer 132 are formed from the bulk layer 130B, and thefirst sub layer 131 contains a larger amount of hydrogen than the secondsub layer 132. Consequently, the first sub layer 131 and the second sublayer 132 may have the same metal element composition, and the first sublayer 131 and the second sub layer 132 may be simultaneously formed byone-time deposition and patterning.

According to an aspect of the present disclosure, the third sub layer133 may be amorphous. For example, the third sub layer 133 may have noC-axis crystallinity. Consequently, the third sub layer 133 may bedistinguished from the first sub layer 131 and the second sub layer 132,and may exhibit excellent electrical properties, whereby the third sublayer 133 may serve as a channel layer. However, the present disclosureis not limited thereto. The third sub layer 133 may have crystallinity.For example, the third sub layer 133 may be crystalline.

According to an aspect of the present disclosure, each of the first sublayer 131, the second sub layer 132, and the third sub layer 133 mayinclude indium (In), gallium (Ga), and zinc (Zn).

Gallium (Ga) is stably coupled to oxygen, and thus has excellent abilityto prevent the permeation of gas. Gallium (Ga) contributes to thestability of a film. In particular, gallium (Ga) may enable the firstsub layer 131 to function as a hydrogen-blocking layer, and may enablethe second sub layer 131 to function as a stable support.

Zinc (Zn) contributes to the stable formation of a film. An amorphousfilm or a crystalline film may be easily formed by zinc (Zn). As aresult, the oxide semiconductor layer 130 may be maintained in the formof a stable film. In particular, zinc (Zn) enables a stable taper shapeto be formed at the edge of the oxide semiconductor layer 130 during aprocess of patterning the oxide semiconductor layer 130. In the case inwhich a stable taper shape is not formed at the edge of the oxidesemiconductor layer 130, hydrogen or other gases may be introducedthrough the interface thereof, whereby the oxide semiconductor layer 130may be damaged.

Indium (In) increases the mobility and the charge density of the oxidesemiconductor layer 130. However, indium (In) is weakly coupled tooxygen. In the case in which hydrogen permeates into the oxidesemiconductor layer 130, therefore, oxygen that has been coupled toindium (In) is coupled to hydrogen instead of indium (In), wherebyoxygen vacancy (O-vacancy) occurs in the oxide semiconductor layer 130.

According to an aspect of the present disclosure, the third sub layer133 includes indium (In), whereby the third sub layer 133 maysufficiently serve as a channel layer. In addition, the second sub layer132 may include indium (In) in order to serve as a channel as well as abarrier. In this case, the first sub layer 131, which has the same metalcomposition as the second sub layer 132, also includes indium (In).

The third sub layer 133, which serves as a main channel layer, has ahigher concentration of indium (In) than the first sub layer 131 and thesecond sub layer 132. Here, the concentration of indium (In) may beexpressed as the content ratio of indium (In) to all metal elements thatare included in each of the first sub layer 131, the second sub layer132, and the third sub layer 133. At this time, the content ratio may beexpressed as atomic percent (at %), which is based on the number ofatoms.

According to an aspect of the present disclosure, since the third sublayer 133 has high mobility and charge density due to high concentrationof indium (In), the third sub layer 133 may serve as a main channellayer.

As previously described, indium (In) is weakly coupled to oxygen. In thecase in which hydrogen permeates into the oxide semiconductor layer 130,therefore, oxygen vacancy (O-vacancy) occurs in the oxide semiconductorlayer 130 due to indium (In). Consequently, the content of indium (In)in the first sub layer 131, which serves as a hydrogen-blocking layer,and the second sub layer 132, which serves as a support, is adjusted toa predetermined range or less, compared to gallium (Ga) and zinc (Zn).

For example, the content of indium (In), gallium (Ga), and zinc (Zn) ineach of the first sub layer 131 and the second sub layer 132 may be setso as to satisfy Equations 1 and 2 below.2≤[Ga]/[In]≤4  [Equation 1]2≤[Zn]/[In]≤6  [Equation 2]

In Equations 1 and 2, [Ga] indicates the number of atoms in gallium(Ga), [In] indicates the number of atoms in indium (In), and [Zn]indicates the number of atoms in zinc (Zn).

In the case in which the content ratio of gallium (Ga) to indium (In) isless than 2 ([Ga]/[In]<2), the hydrogen blocking ability of the firstsub layer 131 may be reduced due to the deficiency in the content ofgallium (Ga). In the case in which the content ratio of gallium (Ga) toindium (In) is greater than 4 ([Ga]/[In]>4), on the other hand, it maybe difficult for a portion of the second sub layer 132 to serve as achannel layer due to the deficiency in the content of indium (In).

In the case in which the content ratio of zinc (Zn) to indium (In) isless than 2 ([Zn]/[In]<2), the film stability of each of the first sublayer 131 and the second sub layer 132 may be reduced due to thedeficiency in the content of zinc (Zn). As a result, a stable taper isnot formed at the edge of each of the first sub layer 131 and the secondsub layer 132, whereby hydrogen or other gases may be introduced throughthe interface between each of the first sub layer 131 and the second sublayer 132 and another layer. In the case in which the content ratio ofzinc (Zn) to indium (In) is greater than 4 ([Zn]/[In]>4), on the otherhand, it may be difficult for a portion of the second sub layer 132 toserve as a channel layer due to the deficiency in the content of indium(In).

Hereinafter, the oxide semiconductor layer 130 will be described in moredetail with reference to FIG. 4.

FIG. 4 is an enlarged view showing part A of FIG. 1.

Referring to FIG. 4, the first sub layer 131 has a larger taper anglethan the third sub layer 133 (θ1>θ3). According to an aspect of thepresent disclosure, the third sub layer 133 includes a larger amount ofindium (In) than the first sub layer 131 and the second sub layer 132,whereas the third sub layer 133 includes a smaller amount of zinc (Zn)and gallium (Ga) than the first sub layer 131 and the second sub layer132. As a result, the third sub layer 133 has a higher etching rate thanthe first sub layer 131 and the second sub layer 132. Consequently, thetaper angle θ3 of the third sub layer 133 is smaller than the taperangle θ1 of the first sub layer 131. Since the third sub layer 133includes a predetermined amount of zinc (Zn) and gallium (Ga), however,the edge of the third sub layer 133 may have a stable taper shape.

As previously described, the first sub layer 131 and the second sublayer 132 are simultaneously formed by patterning. Consequently, thefirst sub layer 131 and the second sub layer 132 may have the same taperangle θ1.

According to an aspect of the present disclosure, the third sub layer133 serves as a channel layer, and a portion of the third sub layer 133may be conductivized in order to contact the source electrode 150 andthe drain electrode 160. More specifically, a portion of the region ofthe third sub layer 133 that does not overlap the gate electrode 140 maybe conductivized. According to an aspect of the present disclosure, theconductivized region of the third sub layer 133 is referred to as afirst conductivized portion 133 a and 133 b. The method ofconductivization is not particularly restricted. A portion of the oxidesemiconductor layer 130 may be conductivized using any of the well-knownconductivization methods. For example, a portion of the oxidesemiconductor layer 130 may be conductivized through selective radiationof argon (Ar) plasma.

Referring to FIG. 4, the third sub layer 133 has a first conductivizedportion 133 a and 133 b formed at the region thereof that does notoverlap the gate electrode 140. Consequently, a contact property of thethird sub layer 133 with the source electrode 150 and the drainelectrode 160 may be improved, whereby the third sub layer 133 maysmoothly serve as a channel layer.

Referring to FIG. 4, the second sub layer 132 has a second conductivizedportion 132 a and 132 b formed at the region thereof that does notoverlap the gate electrode 140 so as to contact the first conductivizedportion 133 a and 133 b. Consequently, the second sub layer 132 may alsoserve as a channel layer.

Meanwhile, the first sub layer 131 contains a large amount of hydrogen,whereby the conductivity of the first sub layer 131 is high. In the casein which the second conductivized portion 132 a and 132 b contacts thefirst sub layer 131, therefore, electrical conduction may be realizedbetween opposite ends of the oxide semiconductor layer 130, whereby theswitching function of the thin-film transistor 100 may not be performed.For this reason, the second conductivized portion 132 a and 132 b isdesigned so as not to contact the first sub layer 131.

In order for a portion of the second sub layer 132 to serve as a channellayer and at the same time for the second conductivized portion 132 aand 132 b, which are formed in the second sub layer 132, not to contactthe first sub layer 131, it is necessary for the second sub layer 132 tohave a predetermined thickness. To this end, the second sub layer 132may have a thickness ranging from 15 to 50 nm. However, the presentdisclosure is not limited thereto. The second sub layer 132 may have athickness of more than 50 nm.

In addition, the second sub layer 132 may have a thickness equivalent to1 to 10 times the thickness of the first sub layer 131. Morespecifically, the second sub layer 132 may have a thickness equivalentto 2 to 5 times the thickness of the first sub layer 131. For example,the second sub layer 132 may have a thickness equivalent to 3 to 4 timesthe thickness of the first sub layer 131.

A gate insulation film 120 is disposed on the oxide semiconductor layer130. The gate insulation film 120 may include at least one of a siliconoxide or a silicon nitride. The gate insulation film 120 may include analuminum oxide (Al₂O₃).

The gate insulation film 120 may have a single-film structure or amulti-film structure. For example, any one of an aluminum oxide layer, asilicon oxide layer, and a silicon nitride layer may individually formthe gate insulation film 120. Alternatively, the aluminum oxide layer,the silicon oxide layer, and the silicon nitride layer may be stacked toform the gate insulation film 120.

Referring to FIG. 1, the gate electrode 140 is disposed on the gateinsulation film 120. Specifically, the gate electrode 140 overlaps atleast a portion of the oxide semiconductor layer 130 and isolated fromthe oxide semiconductor layer 130. The structure of the thin-filmtransistor 100 in which the gate electrode 140 is disposed above theoxide semiconductor layer 130 as shown in FIG. 1 is called a top gatestructure. Here, the oxide semiconductor layer 130 is disposed so as tobe closer to the substrate 110 than the gate electrode 140 on the basisof the gate insulation film 120.

The gate electrode 140 may include at least one of an aluminum-basedmetal, such as aluminum (Al) or an aluminum alloy, a silver-based metal,such as silver (Ag) or a silver alloy, a copper-based metal, such ascopper (Cu) or a copper alloy, a molybdenum-based metal, such asmolybdenum (Mo) or a molybdenum alloy, chrome (Cr), tantalum (Ta),neodymium (Nd), or titanium (Ti). The gate electrode 140 may have amulti-layer film structure including at least two conductive films thathave different physical properties.

The interlayer insulation film 170 is disposed on the gate electrode140. The interlayer insulation film 170 is composed of a insulationmaterial. Specifically, the interlayer insulation film 170 may becomposed of an organic material, an inorganic material, or a stackincluding an organic material layer and an inorganic material layer.

The source electrode 150 and the drain electrode 160 are disposed on theinterlayer insulation film 170. The source electrode 150 and the drainelectrode 160 are connected to the oxide semiconductor layer 130 andspaced apart from each other. Referring to FIG. 1, the source electrode150 and the drain electrode 160 are connected to the oxide semiconductorlayer 130 via contact holes formed through the interlayer insulationfilm 170. More specifically, the source electrode 150 and the drainelectrode 160 are connected to the third sub layer 133 of the oxidesemiconductor layer 130.

Each of the source electrode 150 and the drain electrode 160 may includeat least one of molybdenum (Mo), aluminum (Al), chrome (Cr), gold (Au),titanium (Ti), nickel (Ni), neodymium (Nd), copper (Cu), or an alloythereof. Each of the source electrode 150 and the drain electrode 160may be formed so as to have a single layer made of a metal or an alloyof metals, or may be formed so as to have a plurality of layers, such astwo or more layers.

The oxide semiconductor layer 130, the gate electrode 140, the sourceelectrode 150, and the drain electrode 160 form the thin-film transistor100.

FIG. 5 is a cross-sectional view of a thin-film transistor 200 accordingto another aspect of the present disclosure. Hereinafter, a descriptionof the components that have already been described above will be omittedin order to avoid duplication of description.

Compared to the thin-film transistor 100 of FIG. 1, the thin-filmtransistor 200 of FIG. 5 further includes a light-blocking layer 180 anda buffer layer 121 disposed on the substrate 110. The light-blockinglayer 180 overlaps the oxide semiconductor layer 130.

The light-blocking layer 180 blocks the incidence of light onto theoxide semiconductor layer 130 of the thin-film transistor 200 from theoutside in order to prevent damage of the oxide semiconductor layer 130due to external incident light.

In general, the light-blocking layer 180 is made of an electricallyconductive material, such as metal. For this reason, the buffer layer121 is disposed on the light-blocking layer 180 in order to isolate thelight-blocking layer 180 and the oxide semiconductor layer 130 from eachother. In this case, hydrogen contained in the buffer layer 121 maydiffuse into the oxide semiconductor layer 130, whereby oxygen vacancy(O-vacancy) may occur in the oxide semiconductor layer 130, or the oxidesemiconductor layer 130 may be conductivized.

In order to prevent the occurrence of oxygen vacancy in the oxidesemiconductor layer 130 due to hydrogen or to prevent the oxidesemiconductor layer 130 from being conductivized due to hydrogen, thethin-film transistor 200 according to the aspect of the presentdisclosure includes a first sub layer 131. Specifically, the oxidesemiconductor layer 130 includes a first sub layer 131, a second sublayer 132, and a third sub layer 133. The first sub layer 131 isdisposed to be in contact with the buffer layer 121. Here, the first sublayer 131 is a hydrogen-blocking layer.

FIG. 6A is a cross-sectional view of a thin-film transistor 300according to another aspect of the present disclosure.

The thin-film transistor 300 of FIG. 6A includes a gate electrode 140disposed on a substrate 110, an oxide semiconductor layer 130 disposedso as to overlap at least a portion of the gate electrode 140 andisolated from the gate electrode 140, a gate insulation film 120disposed between the gate electrode 140 and the oxide semiconductorlayer 130, a source electrode 150 connected to the oxide semiconductorlayer 130, and a drain electrode 160 connected to the oxidesemiconductor layer 130 and spaced apart from the source electrode 150.

The structure in which the gate electrode 140 is disposed below theoxide semiconductor layer 130 as shown in FIG. 6A is called a bottomgate structure. Here, the oxide semiconductor layer 130, the gateelectrode 140, the source electrode 150, and the drain electrode 160form the thin-film transistor 300.

Referring to FIG. 6A, the gate insulation film 120 is disposed on thegate electrode 140, and the oxide semiconductor layer 130 is disposed onthe gate insulation film 120. Referring to FIG. 6A, the gate electrode140 is disposed so as to be closer to the substrate 110 than the oxidesemiconductor layer 130 on the basis of the gate insulation film 120. Inthis case, hydrogen contained in the gate insulation film 120 maydiffuse into the oxide semiconductor layer 130, whereby oxygen vacancy(O-vacancy) may occur in the oxide semiconductor layer 130, or the oxidesemiconductor layer 130 may be conductivized.

In order to prevent the occurrence of oxygen vacancy in the oxidesemiconductor layer 130 due to hydrogen or to prevent the oxidesemiconductor layer 130 from being conductivized due to hydrogen, theoxide semiconductor layer 130 includes a first sub layer 131, whichserves as a hydrogen-blocking layer. The first sub layer 131 is disposedto be in contact with the gate insulation film 120.

FIG. 6B is a cross-sectional view of a thin-film transistor 301according to another aspect of the present disclosure. Compared to theoxide semiconductor layer 130 of the thin-film transistor 300 shown inFIG. 6A, the oxide semiconductor layer 130 of the thin-film transistor301 shown in FIG. 6B is configured such that the first sub layer 131 andthe third sub layer 133 are arranged in reverse order. Specifically,referring to FIG. 6A, the first sub layer 131, the second sub layer 132,and the third sub layer 133 are sequentially stacked from the gateinsulation film 120. In contrast, referring to FIG. 6B, the third sublayer 133, the second sub layer 132, and the first sub layer 131 aresequentially stacked from the gate insulation film 120.

FIG. 7A is a cross-sectional view of a thin-film transistor 400according to another aspect of the present disclosure.

Compared to the thin-film transistor 300 shown in FIG. 6A, the thin-filmtransistor 400 shown in FIG. 7A further includes an etch stopper 185disposed on the oxide semiconductor layer 130. The etch stopper 185 maybe made of a insulation material. The etch stopper 185 may protect achannel region of the oxide semiconductor layer 130.

FIG. 7B is a cross-sectional view of a thin-film transistor 401according to another aspect of the present disclosure. Compared to theoxide semiconductor layer 130 of the thin-film transistor 400 shown inFIG. 7A, the oxide semiconductor layer 130 of the thin-film transistor401 shown in FIG. 7B is configured such that the first sub layer 131 andthe third sub layer 133 are arranged in reverse order. Specifically,referring to FIG. 7A, the first sub layer 131, the second sub layer 132,and the third sub layer 133 are sequentially stacked from the gateinsulation film 120. In contrast, referring to FIG. 7B, the third sublayer 133, the second sub layer 132, and the first sub layer 131 aresequentially stacked from the gate insulation film 120.

FIG. 8 is a schematic cross-sectional view of a display apparatus 500according to another aspect of the present disclosure.

The display apparatus 500 according to the aspect of the presentdisclosure includes a substrate 110, a thin-film transistor 100, and anorganic light-emitting device 270 connected to the thin-film transistor100.

The display apparatus 500 including the thin-film transistor 100 of FIG.1 is shown in FIG. 8. However, the present disclosure is not limitedthereto. The thin-film transistors 200, 300, 301, 400, and 401 shown inFIGS. 5, 6A, 6B, 7A, and 7B may be applied to the display apparatus 500of FIG. 8.

Referring to FIG. 8, the display apparatus 500 includes a substrate 110,a thin-film transistor 100 disposed on the substrate 110, and a firstelectrode 271 connected to the thin-film transistor 100. In addition,the display apparatus 500 includes an organic layer 272 disposed on thefirst electrode 271 and a second electrode 273 disposed on the organiclayer 272.

Specifically, the substrate 110 may be made of glass or plastic. In thecase in which the substrate 110 is made of plastic, a flexible displayapparatus may be manufactured. In this case, the substrate 110 mayinclude at least one of polyimide (PI), polycarbonate (PC),polyethersulfone (PES), polyethylene naphthalate (PEN), polyethyleneterephthalate (PET), or polystyrene (PS).

The thin-film transistor 100 is disposed on the substrate 110. Thethin-film transistor 100 includes an oxide semiconductor layer 130, agate electrode 140 disposed so as to overlap at least a portion of theoxide semiconductor layer 130 and isolated from the oxide semiconductorlayer 130, a source electrode 150 connected to the oxide semiconductorlayer 130, and a drain electrode 160 connected to the oxidesemiconductor layer 130 and spaced apart from the source electrode 150.The oxide semiconductor layer 130 includes a first sub layer 131, asecond sub layer 132 and a third sub layer 133, which are sequentiallydisposed. The first sub layer 131 serves as a hydrogen-blocking layer.

Referring to FIG. 8, a gate insulation film 120 is disposed between thegate electrode 140 and the oxide semiconductor layer 130.

A planarization film 190 is disposed on the thin-film transistor 100 inorder to planarize the upper part of the substrate 110. Theplanarization film 190 may be composed of an organic insulation materialthat exhibits photosensitivity, such as an acrylic resin. However, thepresent disclosure is not limited thereto.

The first electrode 271 is disposed on the planarization film 190. Thefirst electrode 271 is connected to the drain electrode 160 of thethin-film transistor 100 via a contact hole formed through theplanarization film 190.

A bank layer 250 is disposed on the first electrode 271 and theplanarization film 190 in order to define a pixel region or alight-emitting region. For example, the bank layer 250 may be disposedat the interface between pixels in a matrix fashion such that the pixelregion can be defined by the bank layer 250.

The organic layer 272 is disposed on the first electrode 271. Theorganic layer 272 may be disposed on the bank layer 250. That is, theorganic layer 272 may not be divided for each pixel, but may becontinuous between adjacent pixels.

The organic layer 272 includes an organic light-emitting layer. Theorganic layer 272 may include a single organic light-emitting layer ortwo or more organic light-emitting layers that are stacked in thevertical direction. The organic layer 272 may emit any one of red,green, and blue light. Alternatively, the organic layer 272 may emitwhite light.

The second electrode 273 is disposed on the organic layer 272.

The first electrode 271, the organic layer 272, and the second electrode273 may be stacked to form the organic light-emitting device 270. Theorganic light-emitting device 270 may serve as a light quantityadjustment layer in the display apparatus 500.

Although not shown, in the case in which the organic layer 272 emitswhite light, each pixel may include a color filter for filtering thewhite light emitted from the organic layer 272 for a correspondingwavelength. The color filter is disposed in a light movement path. In aso-called bottom-emission-type structure, in which light emitted fromthe organic layer 272 moves toward the substrate 110, which is disposedbelow the organic layer 272, the color filter is disposed below theorganic layer 272. In a so-called top-emission-type structure, in whichlight emitted from the organic layer 272 moves toward the secondelectrode 273, which is disposed above the organic layer 272, the colorfilter is disposed above the organic layer 272.

FIG. 9 is a schematic cross-sectional view of a display apparatus 600according to a further aspect of the present disclosure.

Referring to FIG. 9, the display apparatus 600 according to the aspectof the present disclosure includes a substrate 110, a thin-filmtransistor 100 disposed on the substrate 110, and a first electrode 381connected to the thin-film transistor 100. In addition, the displayapparatus 600 includes a liquid crystal layer 382 disposed on the firstelectrode 381 and a second electrode 383 disposed on the liquid crystallayer 382.

The liquid crystal layer 382 serves as a light quantity adjustmentlayer. As described above, the display apparatus 600 shown in FIG. 9 isa liquid crystal display apparatus including a liquid crystal layer 382.

Specifically, the display apparatus 600 of FIG. 9 includes a substrate110, a thin-film transistor 100, a planarization film 190, a firstelectrode 381, a liquid crystal layer 382, a second electrode 383, abarrier layer 320, color filters 341 and 342, a light-blocking unit 350,and an opposite substrate 310.

The substrate 110 may be made of glass or plastic.

Referring to FIG. 9, the thin-film transistor 100 is disposed on thesubstrate 110. The thin-film transistor 100 includes an oxidesemiconductor layer 130, a gate electrode 140 disposed so as to overlapat least a portion of the oxide semiconductor layer 130 and isolatedfrom the oxide semiconductor layer 130, a source electrode 150 connectedto the oxide semiconductor layer 130, and a drain electrode 160connected to the oxide semiconductor layer 130 and spaced apart from thesource electrode 150.

The oxide semiconductor layer 130 includes a first sub layer 131, asecond sub layer 132 and a third sub layer 133, which are sequentiallydisposed. Specifically, the oxide semiconductor layer 130 includes afirst sub layer 131, a second sub layer 132 disposed on the first sublayer 131, and a third sub layer 133 disposed on the second sub layer132. In addition, referring to FIG. 9, a gate insulation film 120 isdisposed between the gate electrode 140 and the oxide semiconductorlayer 130.

The planarization film 190 is disposed on the thin-film transistor 100in order to planarize the upper part of the substrate 110.

The first electrode 381 is disposed on the planarization film 190. Thefirst electrode 381 is connected to the drain electrode 160 of thethin-film transistor 100 via a contact hole CH formed through theplanarization film 190.

The opposite substrate 310 is disposed so as to be opposite thesubstrate 110.

The light-blocking unit 350 is disposed on the opposite substrate 310.The light-blocking unit 350 has a plurality of openings therein. Theopenings are disposed so as to correspond to first electrodes 381, whichare pixel electrodes. The light-blocking unit 350 blocks thetransmission of light through the remaining portion thereof excludingthe openings. The light-blocking unit 350 is not essential, and thus maybe omitted.

The color filters 341 and 342 are disposed on the opposite substrate310, and selectively block the wavelength of light incident from abacklight unit (not shown). Specifically, the color filters 341 and 342may be disposed in the openings defined by the light-blocking unit 350.Each of the color filters 341 and 342 may express any one of red, green,and blue. Each of the color filters 341 and 342 may express a colorother than red, green, or blue.

The barrier layer 320 may be disposed on the color filters 341 and 342and the light-blocking unit 350. The barrier layer 320 may be omitted.

The second electrode 383 is disposed on the barrier layer 320. Forexample, the second electrode 383 may be disposed in front of theopposite substrate 310. The second electrode 383 may be composed of atransparent conductive material, such as ITO or IZO.

The first electrode 381 and the second electrode 383 are disposed so asto be opposite each other, and the liquid crystal layer 382 is disposedbetween the first electrode 381 and the second electrode 383. The secondelectrode 383 applies an electric field to the liquid crystal layer 382together with the first electrode 381.

On the assumption that the surfaces of the substrate 110 and theopposite substrate 310 that face each other between the substrate 110and the opposite substrate 310 are defined as upper surfaces of thesubstrate 110 and the opposite substrate 310 and the surfaces of thesubstrate 110 and the opposite substrate 310 that are opposite the uppersurfaces thereof are defined as lower surfaces of the substrate 110 andthe opposite substrate 310, a polarizing plate may be disposed on eachof the lower surfaces of the substrate 110 and the opposite substrate310.

Hereinafter, the present disclosure will be described in more detailwith reference to Examples, Comparative Examples, and ExperimentalExamples.

Examples 1 to 3 and Comparative Examples 1 to 4

A thin film for forming a bulk layer 130B, having a thickness of 30 nm,was formed on a plastic substrate 110 made of polyimide (PI) by sputterdeposition, a thin film for forming a third sub layer 133, having athickness of 30 nm, was formed on the thin film for forming the bulklayer 130B, and the thin film for forming the bulk layer 130B and thethin film for forming the third sub layer 133 were patterned in order toform a bulk layer 130B and a third sub layer 133. Subsequently, the bulklayer 130B and the third sub layer 133 were heat-treated. As a result,an oxide semiconductor layer 130 was manufactured.

Here, the third sub layer 133 was composed of an InGaZnO (IGZO)-basedoxide semiconductor material including indium (In), gallium (Ga), andzinc (Zn) mixed at a ratio of 1:1:1 based on the number of atoms. Thebulk layer 130B also included indium (In), gallium (Ga), and zinc (Zn).In the bulk layer 130B, however, indium (In), gallium (Ga), and zinc(Zn) were mixed at different ratios based on the number of atoms, asshown in Table 1 below. Thin-film transistors were manufacturedaccording to the composition shown in Table 1 (Examples 1 to 3 andComparative Examples 1 to 4).

TABLE 1 Bulk layer Third sub layer Classification (In:Ga:Zn) (In:Ga:Zn)Example 1 1:3:2 1:1:1 Example 2 1:3:3 1:1:1 Example 3 1:3:4 1:1:1Comparative Example 1 4:1:4 1:1:1 Comparative Example 2 1:1:1 1:1:1Comparative Example 3 1:2:1 1:1:1 Comparative Example 4 1:3:1 1:1:1

Experimental Example 1 Measurement of C-Axis Crystallinity

Photographs of the third sub layer 133 and the bulk layer 130B of theoxide semiconductor layer according to Example 1 were taken using atransmission electron microscope (TEM) in order to check thecrystallinity of the third sub layer 133 and the bulk layer 130B. FIGS.10A and 10B are transmission electron microscope (TEM) photographsrespectively showing the third sub layer 133 and the bulk layer 130B.

Referring to FIG. 10A, it can be seen that the third sub layer 133 hasno crystallinity. On the other hand, referring to FIG. 10B, it can beseen that the bulk layer 130B has C-axis-oriented crystallinity.

Experimental Example 2 Taper Shape

The taper shape of the edge of each of the third sub layer 133 and thebulk layer 130B of each of the oxide semiconductor layers according toExample 1 and Comparative Example 1 was checked using the transmissionelectron microscope.

FIGS. 11A and 11B are transmission electron microscope photographsrespectively showing taper shapes formed at the oxide semiconductorlayers according to Comparative Example 1 and Example 1.

Referring to FIG. 11A, an inverse-taper-shaped end was formed at theedge of the third sub layer 133 of the oxide semiconductor layeraccording to Comparative Example 1. It is determined that theinverse-taper-shaped edge was formed as the result of excessive etchingat the interface between the third sub layer 133 and the bulk layer 130Bdue to the fast speed at which the bulk layer 130 was etched.

On the other hand, referring to FIG. 11B, it can be seen that aforward-taper-shaped end was formed at the edge of each of the bulklayer 130B and the third sub layer 133 of the oxide semiconductor layer130 according to Example 1.

Experimental Example 3 Measurement of Hydrogen Content

The content of hydrogen in each of the oxide semiconductor layersaccording to Examples 1 to 3 based on the depth thereof was measuredusing a dynamic secondary ion mass spectrometer (D-SIMS). The D-SIMS isan apparatus that introduces primary ions having a predetermined amountof energy into the surface of a solid body and analyzes secondary ionsdischarged from the surface of the solid body in order to analyze atomsor molecules constituting the surface of the solid body.

Specifically, the surface of each of the oxide semiconductor layersaccording to Examples 1 to 3 was etched while a predetermined amount ofenergy was applied to the surface of each of the oxide semiconductorlayers, and ions discharged from the surface of each of the oxidesemiconductor layers were analyzed using a CAMECA IMS 7f-Auto, which isa kind of D-SIMS, in order to measure the content of hydrogen in each ofthe oxide semiconductor layers based on the depth thereof (Model:7f-Auto (CAMECA), Source: Cs 10 keV, Sample: −5 keV, Primary Current: 10nA, Raster: 100 μm). The results are shown in FIG. 12.

FIG. 12 is a graph showing the content of hydrogen in each of the oxidesemiconductor layers based on the depth thereof. In FIG. 12, the terms“EX1”, “EX2”, and “EX3” indicate Example 1, Example 2, and Example 3,respectively.

In the graph shown in FIG. 12, the horizontal axis indicates the depth,and the vertical axis indicates the number of hydrogen atoms detectedper unit of time (second; sec), which corresponds to the concentrationof hydrogen.

In the graph shown in FIG. 12, a depth range of 0 to 30 nm correspondsto the third sub layer 133, a depth range of 30 to 47 nm corresponds tothe second sub layer 132, and a depth range of 47 to 60 nm correspondsto the first sub layer 131. Referring to FIG. 12, it can be seen fromExamples 1 to 3 that hydrogen (H) discharged from the substrate 110 wasintroduced into the lower part of the bulk layer 130B, whereby the firstsub layer 131 was formed.

Specifically, referring to FIG. 12, it can be seen that theconcentration of hydrogen in the first sub layer 131 is high, that theconcentration of hydrogen in the first sub layer 131 is abruptly reducedtoward the second sub layer 132, and that the concentration of hydrogenin the third sub layer 133 remains low. The concentration of hydrogen inthe third sub layer 133 is less than 1/10 of the concentration ofhydrogen in the first sub layer 131. It can be seen from the aboveresults that the first sub layer 131 exhibits excellenthydrogen-blocking ability.

FIG. 13 is a graph showing the average content of hydrogen contained inthe third sub layer 133 of each of the oxide semiconductor layers 130according to Comparative Examples 1 to 4 and Examples 1 to 3. In FIG.13, the terms “EX1”, “EX2”, and “EX3” indicate Example 1, Example 2, andExample 3, respectively. In addition, the terms “Comp1”, “Comp2”,“Comp3”, and “Comp4” indicate Comparative Example 1, Comparative Example2, Comparative Example 3, and Comparative Example 4, respectively.

Referring to FIG. 13, it can be seen that the average content ofhydrogen contained in the third sub layer 133 of each of the oxidesemiconductor layers according to Examples 1 to 3 is less than theaverage content of hydrogen contained in the third sub layer 133 of eachof the oxide semiconductor layers according to Comparative Examples 1 to4.

It can be seen from the above results that, in each of the oxidesemiconductor layers according to Examples 1 to 3, hydrogen dischargedfrom the substrate by heat treatment performed at a temperature of 300°C. was introduced into the lower part of the bulk layer 130B, wherebythe first sub layer 131, which is a hydrogen-blocking layer, was formed.

A thin-film transistor including an oxide semiconductor layer 130according to an aspect of the present disclosure exhibits excellentreliability and driving properties. In addition, a display apparatusaccording to an aspect of the present disclosure including such athin-film transistor may have excellent reliability while having a smallthickness.

As is apparent from the above description, a thin-film transistoraccording to an aspect of the present disclosure includes ahydrogen-blocking layer that exhibits an excellent hydrogen-blockingproperty. Even in the case in which an oxide semiconductor layer isformed on a plastic substrate, such as one made of a polyimide (PI), theoxide semiconductor layer is not damaged, whereby the thin-filmtransistor exhibits excellent reliability. Since the thin-filmtransistor exhibits excellent reliability even on a plastic substrate,the thin-film transistor may be usefully used to manufacture a flexibledisplay apparatus.

In addition to the effects of the present disclosure as mentioned above,additional advantages and features of the present disclosure will beclearly understood by those skilled in the art from the abovedescription of the present disclosure.

It will be apparent to those skilled in the art that the presentdisclosure described above is not limited by the above-described aspectsand the accompanying drawings and that various substitutions,modifications, and variations can be made in the present disclosurewithout departing from the spirit or scope of the disclosures.Consequently, the scope of the present disclosure is defined by theaccompanying claims, and it is intended that all variations ormodifications derived from the meaning, scope, and equivalent concept ofthe claims fall within the scope of the present disclosure.

What is claimed is:
 1. A thin-film transistor comprising: a plastic substrate; an oxide semiconductor layer disposed on the plastic substrate and including a first sub layer, a second sub layer and a third sub layer, the second sub layer disposed between the first sub layer and the third sub layer; a gate electrode isolated from the oxide semiconductor layer, the gate electrode overlapping at least a portion of the oxide semiconductor layer; an interlayer insulation film on the gate electrode; a source electrode on the interlayer insulation film, the source electrode contacting an upper surface of the third sub layer of the oxide semiconductor layer through a contact hole; and a drain electrode on the interlayer insulation film, the drain electrode being spaced apart from the source electrode and contacting the upper surface of the third sub layer of the oxide semiconductor layer through another contact hole, wherein the second sub layer has a resistance larger than that of the first sub layer and that of the third sub layer, and a carrier concentration lower than that of the first sub layer and that of the third sub layer, the first sub layer has a hydrogen concentration higher than that of the second sub layer and the third sub layer, each of the first sub layer and the second sub layer has crystallinity, the second sub layer has a thickness equivalent to 2 to 5 times a thickness of the first sub layer, the third sub layer has a first conductivized portion at a region thereof that does not overlap with the gate electrode, the second sub layer has a second conductivized portion at a region thereof that does not overlap with the gate electrode to contact the first conductivized portion, the second conductivized portion does not contact the first sub layer, and wherein the first sub layer of the oxide semiconductor layer contacts the plastic substrate.
 2. The thin-film transistor according to claim 1, wherein each of the first sub layer and the second sub layer has C-axis crystallinity and a wurtzite crystal structure.
 3. The thin-film transistor according to claim 1, wherein the first sub layer and the second sub layer have a same metal element composition.
 4. The thin-film transistor according to claim 1, wherein the third sub layer has no C-axis crystallinity.
 5. The thin-film transistor according to claim 1, wherein each of the first sub layer, the second sub layer, and the third sub layer includes indium (In), gallium (Ga), and zinc (Zn), and the third sub layer has an indium (In) concentration higher than that of the first sub layer and the second sub layer.
 6. The thin-film transistor according to claim 5, wherein a content of indium (In), gallium (Ga), and zinc (Zn) in each of the first sub layer and the second sub layer satisfies Equations 1 and 2 below, 2≤[Ga]/[In]≤4  [Equation 1] 2≤[Zn]/[In]≤6  [Equation 2] in Equations 1 and 2, [Ga] indicates a number of atoms in gallium (Ga), [In] indicates a number of atoms in indium (In), and [Zn] indicates a number of atoms in zinc (Zn).
 7. The thin-film transistor according to claim 5, wherein the third sub layer has a zinc (Zn) concentration and a gallium (Ga) concentration lower than those of the first sub layer and those of the second sub layer.
 8. The thin-film transistor according to claim 5, wherein a zinc (Zn) concentration is higher than an indium (In) concentration in each of the first sub layer and the second sub layer.
 9. The thin-film transistor according to claim 1, wherein the first sub layer has a taper angle larger than the third sub layer.
 10. The thin-film transistor according to claim 1, wherein the third sub layer is disposed to the gate electrode closer than the first sub layer.
 11. The thin-film transistor according to claim 10, wherein the first sub layer is disposed to the plastic substrate closer than the third sub layer on a basis of the second sub layer.
 12. The thin-film transistor according to claim 1, wherein the first sub layer has a hydrogen concentration equivalent to or more than 10 times a hydrogen concentration of the second sub layer.
 13. A display apparatus comprising: a plastic substrate; a thin-film transistor on the plastic substrate; and a first electrode connected to the thin-film transistor, wherein the thin-film transistor comprises, an oxide semiconductor layer on the plastic substrate, the oxide semiconductor layer including a first sub layer, a second sub layer and a third sub layer, the second sub layer disposed between the first sub layer and the third sub layer; a gate electrode isolated from the oxide semiconductor layer, the gate electrode overlapping at least a portion of the oxide semiconductor layer; an interlayer insulation film on the gate electrode; a source electrode on the interlayer insulation film, the source electrode contacting an upper surface of the third sub layer of the oxide semiconductor layer through a contact hole; and a drain electrode on the interlayer insulation film, the drain electrode being spaced apart from the source electrode and contacting the upper surface of the third sub layer of the oxide semiconductor layer through another contact hole, and wherein the second sub layer has a resistance larger than that of the first sub layer and that of the third sub layer, and a carrier concentration lower than that of the first sub layer and that of the third sub layer, the first sub layer has a hydrogen concentration higher than the second sub layer and the third sub layer, each of the first sub layer and the second sub layer has crystallinity, the second sub layer has a thickness equivalent to 2 to 5 times a thickness of the first sub layer, the third sub layer has a first conductivized portion at a region thereof that does not overlap with the gate electrode, the second sub layer has a second conductivized portion at a region thereof that does not overlap with the gate electrode to contact the first conductivized portion, the second conductivized portion does not contact the first sub layer, and wherein the first sub layer of the oxide semiconductor layer contacts the plastic substrate.
 14. The display apparatus according to claim 13, wherein each of the first sub layer and the second sub layer has C-axis crystallinity and a wurtzite crystal structure.
 15. The thin-film transistor according to claim 1, wherein the plastic substrate comprises at least one selected from polyimide, polycarbonate, polyethersulfone, polyethylene naphthalate, polyethylene terephthalate and polystyrene.
 16. A thin-film transistor comprising: a plastic substrate; an oxide semiconductor layer disposed on the plastic substrate and including a first sub layer, a second sub layer and a third sub layer, the second sub layer disposed between the first sub layer and the third sub layer, wherein the second sub layer has a resistance larger than that of the first sub layer and that of the third sub layer, and a carrier concentration lower than that of the first sub layer and that of the third sub layer, the first sub layer has a hydrogen concentration higher than that of the second sub layer and the third sub layer, each of the first sub layer and the second sub layer has crystallinity, the second sub layer has a thickness equivalent to 2 to 5 times a thickness of the first sub layer, the third sub layer has a first conductivized portion at a region thereof that does not overlap the gate electrode, the second sub layer has a second conductivized portion at a region thereof that does not overlap the gate electrode to contact the first conductivized portion, the second conductivized portion does not contact the first sub layer, and wherein the first sub layer of the oxide semiconductor layer contacts the plastic substrate. 